The Digitized Sky Survey is a collection of scanned photographic images from the Palomar Schmidt plates and the UK Schmidt plates. These digital images were used by the Space Telescope Science Institute to create the Hubble Guide Star Catalog (GSC).
It is contained in a set of 102 CD's.

RealSky is a more compressed version of the same data. CDs numbered 1-8 contain images from the Palomar Sky Survey (the entire northern hemisphere, down to about 12 degrees south declination). CDs numbered 9-18 contain images from the UK Schmidt Telescope (UKST) (The entire southern hemisphere up to about 3 degrees north declination.)

"Cartes du Ciel", Sky Charts use a library developed by Bill Gray of Project Pluto and the Windows DLL is based on a version by Chris Marriott of Skymap Software .
You may find all details about this library on their web site and particularly on http://www.projectpluto.com/get_dss.htm and http://www.skymap.com/dss.htm

Change were made to the original DLL code and it was renamed GETDSS.DLL to avoid confusion with the Skymap version.
- Parameters where added for national language CD mount prompt. 
- Add a function to retrieve a simplified list of possible plate including exposure time information.
- Add a function to extract the image from a specified plate. 
I do this two change to put the plate selection in the main Pascal code instead of the C DLL.
- Add DataSource=4 to find plate from North and South RealSky when both are available.
- Change library options for better integration with Pascal.
- Add standard WCS keyword to the output FITS file.

The full source code to build the GETDSS.DLL is available here .
The DLL itself is available from the download page.